This project will determine the efficacy and safety of vitamin D3 supplements for reducing side effects of treatment with aromatase inhibitors in women with breast cancer. The aromatase inhibitors (AI) have become a critical component of adjuvant therapy for breast cancer, but they cause bone pain, joint pain, joint stiffness, and muscle weakness in approximately 40% of patients. These symptoms, referred to as aromatase inhibitor-associated musculoskeletal symptoms (AIMSS), decrease quality of life and medication adherence. Identifying effective treatments for these symptoms may allow for longer and more adherent medication use and thus improve disease-free survival. Because of the role of vitamin D in muscle cell physiology and musculoskeletal pain, there is reason to believe that vitamin D3 may decrease symptoms associated with the use of aromatase inhibitors. Evaluation of the safety of vitamin D3 supplements in women treated with aromatase inhibitors is crucial. In vitro studies have shown that vitamin D3 stimulates the transcription of a gene that increases estrogen production. To assure that vitamin D3 does not abrogate the clinical benefits of aromatase inhibitors, the effect of vitamin D3 supplements on estrogen production and on serum levels of aromatase inhibitor needs to be evaluated. This pilot project will enroll women with breast cancer who experience AIMSS in a double-blind placebo-controlled randomized clinical trial. The trial will compare treatment with vitamin D3 that will achieve and maintain serum concentrations of 25(OH)D between 30-60ng/ml plus 600 mg calcium/day (Experimental group) versus 400 IU D3 plus 600 mg calcium/day (usual care control group). The specific aims of the project are to determine the efficacy of vitamin D3 in reducing musculoskeletal symptoms by comparing the change in symptoms from baseline to 6 months in women who are in the experimental group as compared with the usual care control group, to determine whether supplemental vitamin D3 improves adherence to aromatase inhibitor treatment, and to determine the safety of vitamin D3 by measuring circulating sex hormone levels and serum AI concentration in both groups during the trial This study will generate preliminary data necessary to design a larger, fully-powered multi-centered clinical trial in a future application by our group.